Researchers at The University of Texas at San Antonio have discovered a filtering material that may reduce the environmental cost of manufacturing plastic. The discovery was created by Libo Li, Ruibiao Lin, both post-doctoral students, and Professor Banglin Chen, Dean’s Distinguished Chair Professor of Chemistry and Microsoft President's Endowed Professor at UTSA, along with other scientists at the National Institute of Standards and Technology (NIST) and China’s Taiyuan University of Technology. The scientific advance can extract the key ingredient in the most common form of plastic from a mixture of other chemicals–while consuming far less energy than usual.
The material is a metal-organic framework (MOF), a class of substances that have been used to separate individual hydrocarbons from the organic molecules produced during the oil refining process. MOFs hold crucial value for the plastic and petroleum industries because of this capability, which could allow manufacturers to perform these separations far more cheaply than standard oil-refinement techniques demand.
This promise has made MOFs the subject of intense study at UTSA and elsewhere, leading to MOFs that can separate different octanes of gasoline and speed up complex chemical reactions. However, one major obstacle has been how to extract ethylene—the molecule used to create polyethylene, the plastic used to make shopping bags and other everyday containers. Yet, this particular MOF finding was found so promising that it’s featured today in the prominent journal Science. In the paper, the team shows that a modification to a well-studied MOF enables it to separate purified ethylene out of a mixture with ethane.
Making plastic takes lots of energy. Polyethylene, the most common type of plastic, is built from ethylene, one of the many hydrocarbon molecules found in crude oil refining. The ethylene must be highly purified for the manufacturing process to work, but the current industrial technology for separating ethylene from all the other hydrocarbons is a high-energy process that cools down the crude to more than 100 degrees below zero Celsius.
Ethylene and ethane constitute the bulk of the hydrocarbons in the mixture, and separating these two is by far the most energy-intensive step. Finding an alternative method of separation would reduce the energy needed to make the 170 million tons of ethylene manufactured worldwide each year.
Scientists have been searching for such an alternative method for years, and MOFs appear promising. On a microscopic level, they look a bit like a skeleton of a skyscraper of girders and no walls. The girders have surfaces that certain hydrocarbon molecules will stick to firmly, so pouring a mixture of two hydrocarbons through the right MOF can pull one kind of molecule out of the mix, letting the other hydrocarbon emerge in pure form.
The trick is to create a MOF that allows the ethylene to pass through. For the plastics industry, this has been the sticking point.
A turning point came in 2012, when the creation of the MOF-74 seemed like a good filter for separating a variety of hydrocarbons, including ethylene. UTSA researchers and the rest of the team analyzed previous approaches but an idea from biochemistry finally sent them in the right direction.
“A huge topic in chemistry is finding ways to break the strong bond that forms between carbon and hydrogen,” said UTSA professor Banglin Chen, who led the team. “Doing that allows you to create a lot of valuable new materials. We found previous research that showed that compounds containing iron peroxide could break that bond.”
The team reasoned that to break the bond in a hydrocarbon molecule, the compound would have to attract the molecule in the first place. When they modified MOF-74’s walls to contain a structure similar to the compound, it turned out the molecule it attracted from their mixture was ethane.
The team brought the MOF to the NCNR to explore its atomic structure. Using a technique called neutron diffraction, they determined what part of the MOF’s surface attracts ethane–a key piece of information for explaining why their innovation succeeded where other efforts have fallen short.
“Without the fundamental understanding of the mechanism, no one would believe our results,” Chen said. “We also think that we can try to add other small groups to the surface, maybe do other things. It’s a whole new research direction and we’re very excited.”
(Source: University of Texas at San Antonio)